Magnetically tuned high frequency circuits



May 26, 1942 R. L. HARVEY 2,283,924

MAGNETICALLY TUNED HIGH FREQUENCY CIRCUITS Filed Dec. :51, 1935 2 sneek-sheet 1 l/vvEN'ron Robert L.Harvey ATTORNEY May 26, 1942.

MAGNETICALLY TUNED HIGH FREQUENCY CIRCUITS Filed Dec, 31, 1935 R. L. HARVEY 2 Sheets-Sheet 2 INVENTOR Robert L.Ha,rvey

BY (Mw Peienied Mey 2s, 1942 2,283,924

4UNITED STATES PATENT OFFICE MAGNETICALLY TUNE man FREQUENCY cmcm'rs Robert L. Harvey, Oaklyn, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application DecemberI 31, 1935, serial No. 56,993 24 claims'. (ci. 17a-44) My'invention relates to high frequency comprovided the material is in such form and armunication tuning systems, and more particurangement es to minimize energy consuming eddy larly to adjustable magnetically tuned resonant current and hysteresis losses.

circuits and `inductcrs for operation at radio fre- The most satisfactory cores for coils heretoquencies, 5 fore used in high frequency circuits have con- Heretofore it has been the practice in super- Sisted of extremely finely divided or comminuted heterodyne radio receivers to employ, 'in the inmagnetic material, such as iron Clust, held t0- termediate frequency amplifier, a fixed inductor, Sether With a suitable insulating binder- As un air core type of transformer with a pair of semiexample of extreme fineness of subdivision, it has adjustable screw type condensers with mica dil been considered necessary to use pure iron DOW- electric, mounted on an insulated base, side by del Small enOugh'tO Dass tllI'Ougl'l a 300 mesh side, for tuning the primary and secondary coils screen for use with inductors adapted for the respectively of said transformer. Such a strucbroadcast frequency range 0f 550-1500 kilOCyClesture is shown in De 'rar Patent 2,047,360, dated The cost of production of such nely divided ma- July 14, 1936. Trouble has been experienced, l terial, as by chemical reduction of iron oxide, has after the receivers have been assembled and in been expensive, as Well as the Production 0f the use, with changes in frequency adjustment. Befinished molded COIB- It has been necessary, b8- cause of the, fact that intermediate frequency cause of the low resistance of the iron, t0' provide transformers must be precisely tuned to a given that the particles shall be well insulated elecfixed frequency, a slight change in the capacity trically from each other to reduce eddy Current of the condensers results in detuning of the frelosses; in some cases the particles 0f Dure iron. quency to an extent that necessitates realignor alloy, have been oxidized, and in other cases ment by a service man. Notwithstanding careful an insulating powder 4has been mixed with the precautions in design, it appears to have been imiron powder to minimize electrical contact among possible to prevent such changes occurring in the particles.

the tuning caused by warping of the plates, The problem of manufacturing satisfactory ageing of the dielectric, temperature and moiscores for operation at radio frequencies is quite ture changes, etc. dierent from that of cores for audio frequency In the tuned radio frequency amplifiers, by Way Work. I am aware that iron oxide was proposed of additional example, it has been necessary to many years ago for loading coils in telephony, see accurately matchup inductances of the coils Lee and Colpitts #705,935, but in recent years in cascaded stages, tunable over a band of freworkers skilled in the art have apparently conquencies by means of a plurality of variable considered it necessary to go to the trouble and exdensers ganged for single control, in accordance pense of providing pure iron or alloys and have with the teachings of Giblin #1,842,937 patent. actually taken oxide of iron and reduced it by Various means have been used to vary the invarious processes to pure iron. Whereas Lee and ductance of one or more of the coils in the match- Colpitts disclosed `that ferroso-ferric oxide ing process in production, one means in gen- (FesOr) was suitable for loading coils, they aperal use being the sliding of coil turns as disparently considered it necessary to go tothe closed in the De Tar Patent #1,860,176. An obtrouble of producing it synthetically. Superficial jection to the latter means has been that there tests with the ore magnetite would lead one is no Way to conveniently vary the position of the skilled in the art to assume that the material is turns, which have been cemented in place, e g. unsatisfactory, but, after considerable research I by means of a simple tool, as in the case of trimhave discovered ways and means whereby magmer condensers. Although this method has netite, inexpensive and rather plentiful, may be proven highly useful, a delicate hand operation employed successfully for the purposes disclosed is necessary. To provide structure for a screw without changing chemically the form of the ore. driver type of adjustment, as by means of a For electrical reasons, as Well, I prefer the natvariometer arrangement would entail considervuralmagnetite. able increase in expense Without sufllcient com- It is, accordingly, an object of my invention to pensating advantages. provide a novel and improved inductor tuned For many years it has been known that paracoupling unit for use in radio systems to improve magnetic material operatively disposed in the the gain and selectivity of such systems, and field of an inductor used in radio frequency work factor of merit of circuits therein. produces certain desirable electrical advantages, It is a further object of my invention to provide an improved magnetically tunable transformer or Inductor for use in high frequency communication systems, which, by reason of its novel design, is substantially lower in cost and smaller in size, without a sacrifice in gain and selectivity relative to apparatus heretofore in general use.

A still further object of my invention is to provide an improved resonant coupling unit, avoiding thenecessity for adjustable trimmer condensers, for use in the intermediate frequency amplier of a superheterodyne radio receiver, which, by reason of its novel design, is substantially more stable in regard to the fixed frequency adjustment' throughout the useful life of the receiver, notwithstanding changes in humidity, temperature, and age.

A still further object of my invention is to provide an improved magnetic core, and method of making same, which will have a high permeability and low core loss -when disposed in a varying magnetic field set up by radio frequency currents.

More specifically, it is an object of my invention to provide an improved resonant coupling unit and radio chassis assembly, which, by reason of my novel design, is adapted to low cost production and is characterized by convenient tuning adjustment of the coupling unit in the assembly.

A still further object of my invention is to provide a radio frequency transformer with one or more adjustable permeable core structures for changing the self inductance of one or more of the transformer coils without materially changing the coupling coefficient between coils.

In accordance with my invention, the primary and secondary coils of an intermediate frequency transformer of a superheterodyne receiving system are shunted respectively by small fixed capacitors, although, in some cases, the capacitors are not used, and the coils are adjustably tuned to the desired intermediate frequency by moving molded cores of powdered magnetic material, preferably comminuted magnetite, in the fields respectively of the coils. By reason of my novel design, the transformer coils and the entire shielded coupling unit can be made substantially smaller than air core transformers for the same requirement as to factor of merit, or they can be made larger with a substantial improvement in the factor of merit. The result of using magnetite cores is to reduce the combined cost of the coils and capacitors more than the cost is increased by the addition of the iron cores.

Another advantage is the elimination of adjustable trimmer condensers, the electrical constants of which change considerably with humidity, temperature and ageing.

A' still further improvement is in lower loss tuned circuits (higher Q) which results in higher gain and greater selectivity per amplifier stage, which can be converted into still lower cost and smaller size by reducing the size of the shielding container until the circuit losses are the same as for the standard air core design.

Another advantage is that the weight of the coupling unit is substantially reduced, a reduction of 60% having occurred in certain units made in accordance with my invention. 'I'his is particularly advantageous for radio apparatus used on aircraft.

It is believed that the invention will be better understood from the following detailed description of certain embodiments of my invention and from the accompanying drawings in which Figure 1 is a full sized side elevational view of a portion of a radio chassis having mounted thereon radio apparatus of a superheterodyne system, made in accordance with my invention;

Fig. 2 is a side elevational view in section, drawn to scale, of an intermediate frequency transformer or coupling unit shown in Fig. 1; i

Figs. 3 and 4 are schematic circuit diagrams of portions of a superheterodyne radio system that will serve to illustrate certain applications of my new and improved coupling unit shown in Fig. 2:

Fig. 5 is a side elevational view, partly in section, of a modified form of my radio frequency coupling unit;

Fig. 6 is a circuit diagram of an antenna input circuit for a radio receiving system that will serve to illustrate one of the uses of the coupling unit shown in Fig. 5;

Fig. '7 is an enlarged cross sectional view of an element of the coupling unitA shown in Fig. 2; and

Fig. 8 is an enlarged plan view of the element shown in section in Fig. 7.

Fig. 9 is a modified and preferred form of an intermediate frequency transformer or coupling unit made in accordance with my invention,

Fig. 10 is a bottom view of the transformer unit of Fig. 9 together with a cut-away section of a chassis on which the unit is mounted, l

Fig. 1l represents characteristic curves of my improved apparatus,

Fig. lla is an enlarged view, partly in section, of the coil and core structure of the unit of Figs. 2 and 9 shown in functional relation to the curves of Fig. 11,

Fig. 12 is a view, in side elevation, of a slightly modified form of my invention, corresponding to the coil and core structures of Figs. 2 and 9,

Fig. 13 is a circuit diagram of a still further modification of my invention corresponding to Fig. 12 and illustrating an ideal development thereof, and

Fig. 14 is a view, in side elevation, partly in section, of the molding device used in forming the molded magnetic material in accordance with my invention.

It will be understood, however, that these embodiments of my invention are merely illustrative and that the invention is not limited to these forms.

Referring to the drawings, an intermediate frequency coupling unit, housed in a shielding container or housing I is mounted on the metal chassis 3 of a superheterodyne radio set, preferably with the unit extending through an opening 5 in the chassis. For the conservation of space, the lower portion of the unit extends below the surface of the chassis .and is preferably mounted thereon by means of a clamp 'l which encircles the container I and is clamped thereto by means of a bolt and nut 9. The clamp is provided with a pair of horizontal ange portions Il which are riveted to the chassis at I3 in a manner that will appear obvious. The unit may readily be removed for servicing by disconnecting the outer ends of leads Il, l5, I E and loosening bolt 9. The coupling unit is provided with tuning adjustment screws I1 and I9 at its respective ends, below and above the surface of the chassis respectively. This provides a very convenient and accurate adjusting means, without the use of special tools, for purposes of convenient assembly as well as subsequent servicing, if necessary.

For the reduction of capacity coupling, one of the leads 2| from the unit is brought out from the top and is provided with a clip 23 adapted to engage the grid terminal of a vacuum tube Il. one of the new metal envelope small sized vacuum tubes (Radiotron type #6K"l) being shown. By reason of my novel design, it will be noted that the size of the coupling unit, particularly that portion extending above the chassis, compares favorably with the size of the small vacuum tube. By way of example, the dimensions of the unit are 31/10" long by 1%" in diameter', as compared to x 1'1/8" for the usual air core transformer design. The weight of the new unit is 0.1 lb, as against 0.25 lb. for the air core design. f

Referring more in detail to the interior of the coupling unit, as shown in Fig. 2, the transformer consists of universal sectional wound primary and Qsecondary coils 21 and 29 flxedly mounted on a sleeve 3| of insulating material such as fiber tubing. The tubing is concentrically mounted within the shield container I by means of the end closing plates 33 and 95 respectively. The

substantially smaller overall constructionv of coupling unit, with the same emclency 'as before. This subiect matter is claimed in my divisional Patent 2,180,413, dated .November 21, 1939.

Further, by way of example. the following specifications are given of an intermediate frequency coupling transformer made in accordance with my invention and adapted to operate at 460 kc., although the data is based on a design omitting the shielding innerliner l1, as in Fig. 9, later described. The coils 21 and 29 are each wound with single silk enamelv five strand #40 litz wire in foursections on a ten mil ilber tube Il, inside diameter and 3" long. Each coil is wound in four sections, it having been found that this number gives better practical design than three or five sections, for reducing capacity end plates are provided with eyelets 31 from which depend terminals 99 which in turn support and electrically connect condensers 4| and 43 respectively. The terminal leads of the coils 21 and 29 are also electrically attached to these terminals 39.

end plates each consist of a pair of discs 41 and 49, preferably of laminated synthetic phenol` resin material, or any other suitable insulating material, with a sheet of rubber 5| cemented therebetween. This unit is assembled by applying adhesive material between adjacent surfaces and by applying pressure. For the purpose of retaining the ends of the tubular sleeve 3|, one of the plate sections 49 is provided with a circular slot 53 into which an end of the sleeve 3| is adapted to snugly iit. The function of the rubber sheet 5| is to frictionally grip the adjusting screws I1 and I9 at the threaded bore 51 for insuring against lost motion. The bores 51 in the end plates are threaded, as by a self tapping action by the screws i1 and |9 respectively. The rubber sheet also frictionally grips the coil tubing and prevents turning while adjusting the core.

Referring back to Fig. 2, the transformer coils 21 and 29 are provided with adjustable cores 59 and 6|, respectively, disposed in sliding relation within the bore of the tube 3|. The cores consist preferably of granular magnetite ore. This material, as sifted natural sized sand particles or as granules crushed from larger sized pieces of ore, is mixed with an insulating binder, preferably phenol condensation resin, known as Bakelite, cold-molded under moderate pressure, land cured under moderate temperature, the adjusting screws |1 and I9 respectively being mounted in the ends of the cylindrical cores 59 and 6| respectively.

Referring again to Fig. 2, I have shown an innerlining for the shielding container, prefer- `ably of the same molded magnetic material as The effect of the molded shielding in' construction, or can be used to make possible a and for obtaining other advantages hereinafter described. There are 75 turns of wire per coil section, in eight layers, wound 1/8" wide with spacing, between sections, and 1/2" between inner sections. respectively, of primary and secondary. The iron cores are 2%4" in diameter and "/4" long with a %2 brass screw I1, 3/4" long inserted 's" into the end of the cores. The coil inductances, without iron core, and the fixed capacities should be held to present day production tolerances of i 5%. The iron cores can be held to an eiective inductance tolerance of 11%. With these tolerances provision is desirably made for adjusting the cores to give an inductance change of each coil of around plus and minus 15%.

The cores in the drawings are shown in about the normal intended operating position. It is desirable that the circuit constants be made such that it is not necessary to insert thecores into the support sleeve 3| a distance farther than about mid-way between the innermost adjacent two sections of each of the transformer windings respectively. I have found, in accordance with my invention, that the cores may be adjusted to this position, which I will call the maximum position, to vary the self inductance of each coil without materially changing the coefficient of coupling between primary and secondary. This coupling is largely determined by the inner adjacent sections, respectively, of primary and secondary coils, discussed more fully in. connection with Fig. 11a. In order to insure against travel of the cores beyond this point, various limiting structures may be employed if desirable, although I have not found it necessary to use same in production. By way of example, I have shown an inner sleeve 13 of reduced diameter secured within the sleeve 3| as by any suitable cementing material. Screws l1 and I9 may, if desired, be reduced in length and amount that will limit the travel to the maximum position.

The reason for the avoidance of variable coupling between primary and secondary with different positions of the cores, is that production difficulties are avoided. It is intended that movement of the cores vary only the self inducte ances, respectively, in order to adjust the resonant point of the tuned circuits. The coupling is determined mainlyby the relative spacing of primary and secondary coils on the support sleeve 3| under given conditions of design of the surrounding shield container. For production this coupling is readily predetermined and it is desired that it be a fixed quantity. It can be seen thatvany change in individual units of this coupling. and resulting change in selectivity,

would quite seriously upset this uniformity of production. In other words, the iron cores the maximum position, the inductance is shown to be 1380 micro-henries, whereas with the core removed, the inductance is found to be 530 microlienries. The effective permeability of the core in maximum position is about 2.6. With a one inch length core, the effective permeability was about 3.

As indicated in Fig. 11a, the core should not extend to the left further than the position marked A, the working range being chosen between points A and B. One reason for this' will be seen by referring to curves D, E and F, plotted between percent coupling between primary and secondary, and at the positions of each core, the different curves representing different spacings between primary and secondary.V The primary and secondary cores were shifted equally for each reading. Curve D represents a spacing between innermost sections of respective coils of curve E a spacing of 1/2", and curve F a spacing of The measurements were taken at a frequency of 1000 cycles.

The curves show how critical the spacingis. Curve D shows that a quite ilat coupling curve for the working range may be obtained with a spacing of 5/8. In the structure of Fig. 2, however, a spacing of 1/2 was used, the curve E indicating that the results were satisfactory. Curve F shows that with close spacing between innermost sections of the transformer windings, the percent coupling varied to a greater degree, and to an undesirable degree in the region to the left of the working range.

Allowing for liberal production tolerances of the various elements, the core positions for resonance will lie between points A and B, the desired working range of theiron core, a core travel of about 1%" with the average position at C. In other words, the end of the core is adapted to move between opposite ends of the next to the innermost coil section. This adjustment will give an ind'uctance of about 1060 mlcrohenries (plus and minus 15%), and an average eiIective permeability of two. Referring to Curve Q, it has been found that in the same L/C ratio, the Q or factor of merit, and consequently the gain and selectivity, does not increase appreciably as the core is moved inwardly beyond the mid-point of the coil (point B); the Q rises from seventyeight, core out, to eighty-nine, with core halfway in the coil (point B), and to only ninety-one with iron core in the maximum effective permeability position (point A). This may be accounted for by increased distributed capacity and losses due to the presence of the core. The core and shield material employed in the above examples, is an iron oxide ore sometimes called lodestone, and consists chemically of one part FeO and two parts FezOa. The natural ore is ground, and/or sifted, to the required granular size and magnetically separated from silica and other foreign material. According to Dictionary of Applied Chemistry-Thorpe, vol. 13, 1912 edition, page 378, the following definitions are given of the spinel group, consisting o! magnetic oxide of iron, FeaOi or FeO.FeaO:; an important ore of iron (Fe 72.4 p. c.), Sharply developed crystals with bright faces are not uncommon; these belong to the cubic system and usually have the form of the regularoctahedron or the rhombicdodecahedron. Granular to compact masses are, however, more abundant. The colour is ironblack with a dull, submetallic lustre and a black streak. Sp. gr. 5-l8; hardness 6. The mineral may be always readily recognized by its strong magnetic character; small fragments are picked up by a magnetised knife-blade. Only occasionally are specimens magnetic with polarity (v. Loadstone). As small grains and crystals, magnetite is of wide distribution in many kinds of igneous rocks, especially the darker coloured with a low silica percentage. In such rocks it sometimes forms rich segregations available for mining; as is in the Ural Mountains and at Kirunavara and Gellivara in Swedish Lapland. Other important deposits, e. g. some of those in southern Sweden and Norway, have been formed by the metamorphism of pre-existing iron-ores, where these have been subjected to the baking action of intrusive masses of igneous rock.` Extensive deposits of magnetite are also mined in Fe as FeO Fo as FcgO; FeO; F010.

Magneten (ideal) n.11 48e@ 112. oo Octahedra from Vogclsberg 39-85 1522 1:038 Shepard Mt., Missouri L36 6305 1147-02" By commercial analysis I have found that the magnetite giving best results comes from the Adirondacks and is the ideal compound as listed in Mellor and described as FeaOi or FeO, FezO: (Fe 72.4%) in Thorpe. It is noted that the reference Fe 72.4% in Thorpe (which is the percentage of iron by atomic weight) is the same as the combined iron percentage in Mellor, i. e. 24.11-|4829=72.4%. The ratio of Fe as in FeO and Fe as in FezOs is 1:2. The ratio by weight of FeO to Fe2O3 is about 31:69.

Iron particles size.-The required neness of magnetite particles is determined by: first, desired permeability and permissible loss, (the finer particle cores have lower loss but lower permeability); secondly, mechanical strength and appearance, (finer particles will make stronger and smoother surface cores). As between permeability and losses, there is an optimum compromise for a certain frequency range. The larger particles give higher permeability because a given mass of ore material is more compact under the conditions as formed in nature than when particles are present and molded with a binder synthetically.

For frequencies around 460 kilocycles, I find that iron particles passing 40 mesh and held back on 60 mesh screens make the best compromise cores, The iron particle size is not very critical over fairly wide ranges of operating frequencies. Thus, cores designed for 460 kc.

particles together, also as an insulator betweenI magnetic particles, and probably as a lubricant to allow the particles to slide closely together during molding. It should be noted that the same material is used as binder, insulator, and lubricant. For this I prefer to use Bake1ite, a resinous phenol condensation product, preferably starting with uncured Bakelite in powder if' form and adding a solvent. A synthane Bakelite varnish in liquid form may, if desired, be used. The proportion of binder and iron varies with size of iron particles and molding process. I have found that a mixture in the ratio of one part by volume of binder'to fourteen parts of 40-60 mesh magnetite makes satisfactory cores, according to my invention.

Molding process. The preferred molding process consists of (a) Mixing fourteen parts magnetic particles and one part dry Bakelite powder in a-mill. (b) Adding about four parts of the above mixture, by volume to one part of a liquid solvent such as acetone, As the mixing process is continued, part of the binder solvent will evaporate, and the mass will in time break up and return to a granular mixture, leaving a dry coating of Bakelite insulation on the iron oxide particles; (c) pouring the coated magnetite (like sand) into the hopper of the mold and applying about three tons pressure to the mold for cold molding (Fig. 14); (d) Removing the cores, cold molded, and placing in an oven, i

and curing the cores at 150 C. to 200 C. for about 2 hours.

'I'he correct pressure used in molding magnetite is an important and critical factor in the production of a core having satisfactory characteristics for radio frequency work, as are some of the other factors involved in my process, as will be seen from the following: Whereas it is desirable to employ a very high pressure in molding in order to increase the density of the magnetic material in order to correspondingly increase the permeability, too great a pressure causes the magnetic particles to break through the insulation material with which it is mixed, resulting in increased eddy current loss. Too low a pressure, and resulting lower density of magnetic particles, produces a permeability that is too low for satisfactory results. I have found, however, that because of the relatively high electrical specific resistance of magnetite as compared to pure iron, a much greater amount of points of electrical contact are allowable between particles without unduly increasing losses. This means that a greater density of material may be used.

If too much binding material is used the excess binder displaces magnetite particles and results in a loss of permeability. On the other hand if too small an amount is used there results a poor insulation between magnetic particles and increases losses.

If the mesh size is too small, there is not enough magnetic material in a given volume, and there is a resulting lowering of permeability. Nature has compressed a maximum amount of magnetic material in a given piece and it appears difficult' to duplicate this synthetlcally by pressing together many fine particles. It is, therefore, de-

sirable, from the point of view of permeability, to employ natural particles as large as possible, compromising with eddy 4current losses. If the particles are too large the eddy current losses increase for well known reasons.

Referring to Fig. 14, I have found that a better core is made by the use of a double end pressure mold. A mold sleeve 95 forms with a base plunger 96 a hopper into which the magnetic mixture is poured for forming the core 59 with the screw insert i1. An upper plunger 91, havinga recess to accommodate the screw I1, fits snugly into the upper end of sleeve 95. The sleeve fits snugly around the plungers and is free to move with respect to both plungers as the pressure is applied to the upper e'nd of plunger 91, thereby resulting in a compressed core of uniform density throughout. 'I'he inner lining for the shield is` made in the same manner as described for the core, with suitable changes made in the mold. 1

The adjusting screw may be molded in the cores initially, or the cores may be drilled out and the screw inserted, and cemented with a drop of collodion or liquid Bakelite after one hour of heat treatment, followed by a second heat treatment of one hour.

Referring to Fig. 3, I have shown diagrammatically the circuit of my improved intermediate frequency coupling unit with its primary coil 21 connected in the plate circuit of a first detector thermionic tube 15 and with its secondary 29 connected in the input grid of an intermediate frequency amplifier tube 11. In circuits of this nature, it is desirable that capacity coupling between primary and secondary be at a minimum. It has been found that high capacity coupling disturbs the characteristic response curve in some amplifiers. The coils are coupled preferably with optimum coupling. Whereas in transforme:` units, heretofore used for circuits of this type there existed a substantial amount of undesirable coupling between the trimmer condensers mounted side by side on the transformer base. The condensers 4l and 43 are remotely mounted at opposite ends of the unit as shown in Fig. 2 and are, in addition, of substantially smaller size by reason of their being of the fixed, nonadjustable, type.

Referring to Fig. 4, I have shown diagrammatically an application of my improved coupling unit operatively disposed between the last intermediate frequency amplifier 19 and a diode detector 8|. It has been well known for some time that, in using an intermediate frequency diode detector and/or automatic volume control rectiiler with inherent capacity coupling between primary and second circuits, a reduction in the size of the by-pass capacitor 83 in the secondary circuit across the diode load resistor 85 for improving audio fidelity renders it impossible to obtain a satisfactory symmetrical resonance re sponse curve'l This condition is made still mor.3 unsatisfactory if the diode transformer secondary 29 is tapped, as shown, for the purpose of obtaining greater selectivity by reducing the load on the transformer. I have found that, by reason of my improved construction wherein the condensers 4| and 43 of Fig. 2 are small in dimension and are mounted at remote ends of the unit, the inherent undesired capacity is substantially reduced. Furthermore, bringing the leads out at opposite ends in the manner shown, also reduces the capacity as do other novel features of the design. The result is that a symmetrical resonance response curve is obtained in using my device in the circuit of Fig. 4 even when capacitor 33 is much smaller than used in previous design. Certain of these features are claimed in my divisional Patent 2,180,413.

Referring to Fig. 5, I have shown a more simplified device. The inner shield lining of molded magnetite is preferably molded under pressure within the shielding container, forming therewithy a unitary construction, although it is also quite'desirable to mold the shield lining separately, as in Fig. 2, and cement it is place, as by collodion. 'I'his construction is provided with end support plates 33 and 35 as in the case of Fig. 2, although only one adjustable core of magnetic material is employed. 'I'he plate 35 is held in spaced relation by means of a spacer 31 of insulating material, the ends of the shield container being bent over at 34 to secure the parts in place. The coil 39 may represent a single inductor or a primary and secondary respectively of a broadly tuned radio frequency or intermedi-v ate-frequency transformer whereinv the distributed capacity of the coils is used instead of physical condensers as part of the tuned circuit. In some arrangements it is desirable that the secondary be wound over the primary coil to insure maintaining constant coupling regardless of movement of the core. In such a case, the inner layers of coil 99 constitute the primary and the outer circumferential layers the secondary. A structure of this nature may be employed as a diode driving transformer in the circuit of Fig. 4, the condenser 43 being omitted, if desired, from the secondary circuit.

Fig. 6 represents diagrammatically an antenna input circuit wherein the unit in Fig. may be used to advantage. In this case the core is employed to give a fine adjustment of the inductance of the secondary 99 in order to match its inductance with that of the inductances of succeeding stages adapted to be tuned to the same frequency. It may also be employed to obtain the desired inductance adjustment of coil 99 in relation to the inductance of a superheterodyne oscillator, the tuning condenser of the oscillator being ganged with the tuning condenser 9| in the antenna circuit. Such an arrangement obviates the necessity of adjusting turns, the usual practice as explained in the ilrst part of the specification, besides giving a very substantial increase in the gain of the antenna input circuit due to the lower resistance of the circuit. This arrangement has been found to be particularly useful for application to automobile radio receivers where it is desirable to obtain maximum gain in'the antenna circuit.

Referring to Figs. 9 and 10, a modified form of my invention as shown in Fig. 2, the shield container |00 is made square, in section, with rounded corners. The upper end has the edges bent over at |0| to form a flange against which the top insulating support plate |03 abuts. The lower end of the shield is provided with a pair of bolts |05, riveted to the container at |09, for securing a bottom insulating plate |01 in place as well as the entire inside coil assembly, with the aid of nuts |09. 'I'he plate |01 lies within the walls of can |00 except for outwardly extending ears |00 which engage cut away sections of g the shield as an abutment. 'I'he free ends of the bolts, extending beyond the nuts |09, are adapted to protrude through apertures in a radio chassis,

i|3 for securing the unit in place on the chassis 75 with the aid of nuts in' registry with an opening H5 therein for access to terminals and adjusting screw.

A threaded bushing |2, as of brass, is mounted centrally in an opening in each support plate |03 and |01 and is secured in place as by a staking or upsetting operation at ||1. The bushings serve to carry the adjusting screws I1 and I9 for the cores. To prevent binding of the cores in the coil support sleeve 3|, in case of slight inaccuracies of alignment, the threaded portion of the bushing is limited in length and is remote from the sleeve 3|, thereby permitting a little side play. Tight frictional action between the screw I9, for example, and the bushing threads isobtained by means of an angular spring ||9 which engages the screw I9 through a slot |2| in the bushing exposing the screw, and the back of the bushing, thereby forcing the screw against the threads in one side of the bushing.

The bushings .also serve to support the sleeve 3| with a force fit at its ends over shoulders |23, respectively of the bushings. The shoulders are longitudinally knurled at |24 for purposes of giving a tight fit and a strip of paper or fiber |29 is disposed around the sleeve 3| at each end to reinforce it where it is forced on over the shoulders |23.

Terminals |21 are disposed in special apertures |29 in each of the end support plates, and are held in place by deforming laterally the projections I 3|. The terminals are adapted to have soldered to their inwardly extending portions leads from the inside coils 21 and 29 (Fig. 2); certain of them carry the condensers 4i and 43 as in Fig. 2. External leads |35 are soldered to the external ends of the terminals. A cap |33 is disposed over the upper end of the shield |00 for minimizing capacity coupling to the otherwise exposed terminals on the top of the insulation plate |03.

Referring to Fig. 12,*while for production it is desirable to have uniform spacing between coil sections, the inner adjacent sections 23 and 33 of primary and secondary coils 21 and 29 may be spaced a greater amount from their other associated coil sections in order to further increase the independence of adjustment of the self inductances with respect to coupling which it is desired to keep constant at a predetermined value. This permits of a greater insertion of the core into the remaining coil sections without exceeding vthe permissible e'ect upon coupling between primary and secondary. It is desirable that the adjacent coil sections 23 and 3l be at the low radio frequency potential ends of the primary and secondary, respectively, to reduce capacity coupling. The same is true of Fig. 2.

A further desirable modification of my invention is illustrated in Fig. 12. A coupling coil 34 0f a few number of turns, is wound between the first and second sections, at the high radio frequency potential end of the primary coil. For this position itc'an be seen that the mutual coupling between the primary coil 21 and the coupling coil 34, will remain unchanged while thel molded core is moved over the working range as shown in Fig. 11a. In accordance with the teachings of Carlson Patent 1,871,405, a switch means 36 is provided for throwing the coil 34 in or out of the circuit to control broadness of the resonance curve of the transformer circuit. A suitable value of resistance 33 is provided in series with the coupling coil, as in the Carlson patent, to prevent double peaked response curve.

Carrying this feature further to obtain a maximum independence of inductance adjustment by cores B9 and il with respect to the coupling, I have shown in Fig. 13,'coupling coil sections 28 and ll magnetically isolated from the remaining sections. They are coupled together, and are preferably provided with an auxiliary core il which may be adjustable. The coupling between circuits is determined mainly by sections 2l and 30 which may be isolated by placing them in a separate shield, or more inexpensively. in the same container but mounted at right angles as indicated.

I claim as my invention:

1. A pair of resonant coupled circuits for an intermediate frequency amplifier of a superheterodyne receiving system, comprising a pair of fixed inductors, having turns disposed in extended relation, a relatively adjustable core of magnetically permeable material for eachinductor, said inductors being loosely coupled and relatively so disposed that certain respective turns are adjacent and other turns are remotely positioned, said cores being disposed for operation in the field of said remotely positioned turns for varying the self inductance of said inductors without substantially changing the coefficient of coupling.

2. The invention as set forth in claim l characterized in that a coupling coil is tightly coupled to said remotely positioned turns of one of said inductors, and means for connecting and disconnecting said coupling coil at will in series with the other of said inductors for controlling the broadness of resonance.

3. 'I'he invention as set forth in claim 1 further characterized in that said variable means comprises a body of molded granular magnetite.

4. In a radio receiving system, a plurality of resonant circuits comprising respectively inductance coils o f fixed nature and variable condensers, subject to common control, for tuning said circuits over a band of wave lengths, means for adjusting the inductance of one or more of said coils to a predetermined value relative to the inductance of the other of said coils for effecting a desired tracking relation of said circuits in selectively tuning through said band, said means comprising an adjustable magnetic core of molded magnetite disposed in the field of one yof said coils.

5. A plurality of coupled circuits for an intermediate frequency amplifier of a superheterodyne receiving system, comprising a pair of xed inductors having remote and adjacent turns, respectively, a fixed capacitor connected to one of said inductors and forming therewith a resonant circuit, an adjustable paramagnetic core disposed in operative relation with respect to the remote turns of said last named inductor for tuning said last named circuit to said intermediate frequency without substantially effecting coupling between circuits, said core being characterized by finely comminuted particles of magnetic material molded with a binder and constituting the sole means for adjustably tuning said last named circuit to said intermediate frequency.

6. In a radio frequency coupling system, a plurality of coupled circuits comprising a pair of magnetically coupled inductors, said inductors having turns positioned relatively remote magnetically and turns relatively closely coupled, respectively, a magnetically permeable core for at least one of said inductors, said core being ad- {(lie coefncient of coupling Justably disposed for operation'substantiaily entirely in the field of the remotely positioned turns thereof, whereby the self-inductance of said one of said inductors may be independently adjusted a desired value without substantially affecting between said circuits.

'7. The invention as set forth in claim 6 wherein said closely coupled turns are in substantially isolated magnetic coupling relation relative to said remote turns.

8. The invention as set forth in claim 6 wherein a separate core of magnetically permeable material is disposed in the field of at least one of said relatively closely coupled turns.

9. The invention as set forth in claim 6 wherein a separate core of magnetically permeable material is adjustably disposed in the field of at least one of said relatively closely coupled turns for varying the coupling.

10. The invention as set forth in claim 6 wherein said `closely coupled turns are substantially isolated magnetically from the other turns and a. separately adjustable magnetic core is provided for varying the coupling.

1l. The invention as set forth in claim 6 wherein at least one of said inductors is composed of'three or more substantially unequally spaced sections of turns coaxially arranged, characterized in that the greater spacing separates said closely coupled and said remotely coupled turns.

12. In a radio frequency coupling system, a plurality of coupled circuits comprising a pair of magnetically coupled inductors, a magnetically permeable core adjustably disposed in the field of at least one of said inductors for varying the self-inductance thereof to a desired value without substantially changing the coupling, and a separate core of magnetically permeable material adjustably disposed relative to said inductors for varying the coefhcient of coupling therebetween. f

13. A high-frequency coupling device including plural inductance coils and capacitors connected to provide a pair of coupled resonant circuits, and ferromagnetic cores adjustable relatively to said coils in such a manner that the selectivity of each of said circuits is maintained constant, said coils and said cores being so disposed that movement of said cores relative to said coils produces simultaneous and substantially proportional changes in the square root of the product of the effective inductances of said circuits and in the mutual inductance between said circuits.

14. A high-frequency coupling device including plural inductance coils and capacitors connected to provide a pair of coupled resonant circuits, and ferromagnetic cores adjustable relatively to said coils in such a manner that the selectivity of each of said circuits is maintained constant, said coils and said cores being so disposed that movement of said cores relative to said coils alters the resonant frequency of said circuits while maintaining the degree of coupling between said circuits substantially constant.

15. A high-frequency coupling device including plural inductance coils and capacitors connected to provide a pair of coupled resonant circuits, and ferromagnetic cores adjustable relatively to said coils in such a manner that the selectivity of each of said circuits is maintained constant, said coils and said cores being so disposed that movement of said cores relative to said coils alters the effective inductance of said circuits while maintaining the degree of coupling between said circuits substantially constant.

16. A high-frequency coupling device including two groups of inductance coils, plural capacitors. said coils and said capacitors being connected to provide a pair of inductively coupled resonant circuits, ferromagnetic core means fixedly positioned relatively to a first of said groups, and adjustable ferromagnetic core means movable relatively to the second of said groups, said coils and said core means being so disposed that movement of said adjustable core means relative to said second group produces simultaneous and substantially proportional changes in the square root of the product of the effective inductances of said circuits and in the mutual inductance between said circuits.

17. A high-frequency coupling device including two groups of inductance coils, plural capacitors, said coils and said capacitors being connected to provide a pair of coupled resonant circuits, ferromagnetic core means iixedly positioned relatively to a first of said groups, and 'adjustable ferromagnetic core means movable relatively to the second of said groups. said coils and said core means being so disposed that movement of said adjustable core means relative to said second group alters the resonant frequency of said circuits while maintaining the degree of coupling between said circuits substantially constant.

18. A high-frequency coupling device including two groups of inductance coils, plural capacitors, said coils and said capacitors being connected to provide a pair of coupled resonant circuits, ferromagnetic core means f'lxcdly positioned relatively to a first of said groups. and adjustable ferromagnetic core means movable relatively to the second of said groups, said coils and said core means being so disposed that movement of said adjustable core means relative to said second group alters the effective inductances of said circuits while maintaining the degree of coupling between said circuits substantially constant.

19. A high-frequency coupling device including first and second resonant circuits, first and second inductive windings in each of said circuits, ferromagnetic core means flxedly positioned within said first windings, and adjustable ferromagnetic core means positioned within but movable relatively to said second windings, said windings and said core means being so disposed that movement of said adjustable core means relative to said second windings produces simultaneous and substantially proportional changes in the square root of the product of the effective inductances of said circuits and in the mutual inductance between said circuits.

20. A high-frequency coupling device including first and second resonant circuits, first and second inductive windings in each of said circuits, ferromagnetic core means fixedly positioned within said first windings, and adjustable ferromagnetic core means positioned within but movable relatively to said second windings, said windings and said core means being so disposed that movement of said adjustable core means into said second windings increases the effective inductances in said circuits and increases the coupling between said circuits in such a way as to maintain the selectivity of said coupling device substantially constant.

21. A high-frequency coupling device including first and second resonant circuits, first and second inductive windings in each of said circuits, ferromagnetic core means ilxedly positioned Within said first windings. and adjustable ferromagnetic within but movable rela.-

quency oi' said circuits while maintaining the degree of coupling between said circuits substantially constant.

22. A high-frequency coupling device including first and second resonant circuits, first and secd inductive windings in each of said circuits, ferromagnetic core means ilxedly positioned within said first windings, and adjustable ferromagnetic core means positioned within but movable relatively to said second windings, said windings and said core means being so disposed that movement of said adjustable core means relative to said second windings alters the effective inductances of said circuits while maintaining the degree of coupling between said circuits substantially constant.

23. A selective high-frequency coupling device including in combination input and output resonant circuits each having an inductor and a xed capacitor and intended for operation at a given frequency, means for adjusting the inductance of each of said circuits to compensate for changes in the capacitance thereof due to conditions of use to thereby maintain resonance at said frequency, said circuits being so coupled as to maintain the over-all gain of said coupling device substantially constant regardless of said inductance adjustment.

24. A selective high-frequency coupling device including in combination a pair of coupled resocapacitor and intended for operation at a given frequency, movable ferromagnetic cores for adjusting the inductance of each of said circuits to compensate for changes in the capacitance thereoi.' use to thereby maintain resonance at said frequency, the coupling between said circuits being varied by said cores in such a way as to maintain the over-all gain of said coupling device substantially constant regardless of said inductance adjustment.

' ROBERT L. HARVEY. 

